This invention pertains to structural arrangements to reduce the overhung load on cantilever rig supporting structure related primarily to platforms that float or stand on legs with limited load supporting ability.
Cantilever rig structures evolved from the need to drill more wells from offshore platforms and there is a need to provide means to change the load arrangement to reduce the effect of deadload extended from the side of platforms.
Some definitions are now in order. As used herein, the well centerline coincides with the hook lift axis. In changing the relationship between the well axis and structural features, the structure accepts the changes, assuming the well centerline is already defined. Things to be changed to accomplish the objects of the invention are defined, all else essential to the related drilling activity is encompassed by the terms platforms and drilling rigs. The installation alluded to is a jack-up rig with three legs as a matter of convenience. The invention is applicable to any installation that uses cantilevered beam arrangements, extending over the side, to support a drilling rig. Rig floor, as used herein, is synonymous derrick floor and drilling floor.
Offshore platforms are designed for particular vertical loads and those loads determine the required load supporting ability of the individual legs. The legs involved are usually columns driven into the sea bed or vertically movable legs of jack up rigs. When loads are supported on cantilever structure extended from one side of the platform the legs nearest the load center of gravity usually define the load supporting ability. The center of gravity of the composite load can be defined as being made up of indivisible load elements times their distance extended from the platform, as measured between gravity and vertical support vectors. In a practical sense the indivisible loads consists of such as draw works, hook loads, dynamic brakes, reels of spare line, racks of standing pipe, and the like.
When the need to accept more cantilever supported load than originally planned by design is realized, the structure under the load has to be strengthened. The structure added also adds to the load on the critical platform legs. When the support ability of the critical legs is approached, any dead load increase caused by added reinforcing of the extending platform reduces the total hook load carrying ability of the overall system.
Distinguishing between productive loads and static loads, racked pipe loads, hook load, draw works weight, and the like can be considered productive loads. The structure added to support those loads can be considered static loads. Both these defined type loads are extended over the side of the platform. These loads tend to lift the distant legs and add that lift load to the downwardly directed load on the nearest, most critical, legs.
The effect of extended loads is proportional to the distance of the specific load from the edge of the platform. Placing the productive load nearer the platform reduces the needed static structure, and its static weight, as well. The benefit is cumulative.
Some dimensions are fixed, such as the distance of the hook load vector relative to the platform. That distance is fixed by the well bore position relative to the platform. When the conventional rig was modified to extend from the conventional platform, the derrick structure was not modified to achieve the fixed load requirements while minimizing the overhung load effects. There are no fundamental requirements that dictate a symmetrical derrick structure that evolved with the drilling industry. The well bore is conventionally in the geometric center of the derrick floor plan, and that is not necessary.
The arrangement of draw works, tubular goods racking structure, and the vee door through which new tubular lengths are drawn into alignment with the well bore have to be in view of the driller. That requirement defined rig floor layouts that evolved under symmetrical derricks. When the derrick was extended over the side of the platform, the original layout was accepted, and design factors took the current loads into consideration. Those current loads were factored into the design of the platforms and cantilever structure. The system was, at that time, satisfactory. Deeper drilling, increased drill string diameter, and multiple completions added loads not originally expected. With added loads, overload problems arose. The present invention addresses changes from conventional derrick shape and rig floor layouts to reduce the effect of overhung loads.
The rig floor plan is changed to place the vertical hook lift axis about one third of the floor length from the aft end of the floor. Compared with the usual thirty foot square rig floor with a centered hook lift axis, that change moves the permissible well center line five feet farther aft. With a given distance between a well centerline and the aft end of the platform hull, the rig floor can be moved five feet toward the hull. The result is a greatly reduced bending moment on the cantilever beams supporting the rig. The draw works, and it""s related dynamic brake, are moved from the aft end of the rig floor to the port side of the floor, somewhat more toward the hull. The drillers position and related protective structure is between the draw works and the aft end of the rig floor, positioned to allow the driller a view of the traveling block path, vee door, and set back area. The set back area, increased by aftward movement of the hook lift axis, is divided and placed on opposite sides of the vee door which is generally centered, and unchanged, on the fore side of the derrick.
The derrick is asymmetrical in the central longitudinal vertical plane, with the crown block arranged to place the hook lift axis as stated. The port and starboard sides of the derrick are arranged to generally support the vertical stands of pipe in the set back areas.
The spare line spool support is moved from the rig floor to the hull.
The object of this invention is to reduce the bending moment on the cantilever beams supporting a drilling rig.
It is another object to move the hook lift axis, relative to the rig floor, to allow the rig floor to be moved toward the supporting platform to reduce bending moment on the cantilever beams.
It is yet another object to rearrange heavy machinery on the rig floor to place the heavier portions toward the platform to reduce bending moment on the cantilever beams.
It is still another object of the invention to increase the reach of the drilling system to place the platform farther from the most distant well centerline.
These and other objects, advantages, and features of this invention will be apparent to those skilled in the art from a. consideration of this specification, including the attached claims and appended drawings.